This invention relates to a device for preparing a chemically reacting mixture and for ejecting the mixture into a mold cavity, the mixture comprising at least two components, in particular a mixture of isocyanate and a polyol that reacts completely to form polyurethane. The device comprises a mixing head having an outwardly open bore, and a reciprocable ram coupled to the head, the ram having an ejector rod axially movable in the bore for therewith defining a mixing chamber and for ejecting the mixture from the chamber.
The mixing head has opposed infeed nozzles for respectively injecting the isocyanate and a polyol into the bore. Mixing results from the impact of one chemical on the other. In its retracted position, the rod defines a mixing chamber with the bore and is positioned upstream of the infeed nozzles.
The degree of mixing depends on the mixing pressure, i.e., the Pressure which exists in the mixing chamber during the mixing of the components.
German Pat. No. 20 65 841 discloses a device as generally characterized above which has a baffle located at the downstream end of the mixing chamber below the inlet nozzles, the baffle being in the form of a rod reciprocable along its longitudinal axis for regulating the mixing chamber pressure depending on the extent of the projection of the baffle into the chamber. After the mixture is prepared, the baffle rod is retracted out of the bore to permit the ejector rod to axially extend through the bore for ejecting the mixture into the mold in which molded bodies of polyurethane are formed. During its reciprocation, the ejector rod cleans the bore and the mixing chamber of any remnants of the chemically reacting mixture, and is therefore known as a cleaning rod.
In published German patent application No. 30 22 132.1, a baffle is located at the downstream end of the mixing chamber below the infeed nozzles, the baffle being in the form of a cylindrical body rotatable about its central axis, and the rod having a transverse opening coaxial with the mixing head bore and chamber. In the extended position of the ejector rod, the transverse opening aligns with the mixing head bore. During preparation of the mixture upon inletting the two chemically reacting components, the barrier rod is rotated about its axis for shifting its opening a predetermined angle relative to the mixing head bore for regulating the chamber pressure.
Another similar device is disclosed in German Pat. No. 23 27 269 in which the mixing head has perpendicularly related bores from which the mixture is ejected by the ejector rod. The mixing chamber is located at the juncture of the perpendicular bores. A reciprocable bearing rod is likewise located at this juncture such that the mixture impacts against the rod before it is ejected. The angled path of the bore considerable reduces the speed at which the mixture can be ejected.
Still another device is disclosed in German Pat. No. 33 40 889 in which the mixture is divided into several partial streams downstream of the mixing chamber, some of which are accelerated and some of which are restrained and then again recombined. This is intended to effect improved mixing and a shorter retention time over the reacting mixture in the mixing head with simultaneous homogenization. In carrying out the Procedure of this known device, a pair of slides are provided rotatable about their common axis and each having transverse openings provided in the flow direction of the mixing chamber. During the mixing of the components, these openings define so-called overflow channels which effect the division of the mixture flow into accelerated and retarded partial streams as the transverse openings enclose a fixed angle relative to the axis of the mixing head bore in which the ejector rod reciprocates.
In all these known devices, the baffles or double slides are shifted to a predetermined position below the mixing chamber at the commencement of mixing in which they remain until mixing is completed, and are then returned to a position in which the ejector rod can be extended without obstruction for ejecting the reacted mixture. Subsequently, after completing the cleaning phase, the ejector rod is retracted above the mixing chamber, while the baffle or double slide is again shifted into its working position during mixing.
Thus, in these known devices, the mixing pressure is regulated in a limited manner by the specific setting of the reciprocable or rotatable baffle rods, and the size of the transverse opening thereof is set in accordance with the diameter of the mixing head bore and the mixing chamber in which the ejector rod reciprocates. Thus, a presetting of the mixing pressure for the mixture is generally not made possible by these known devices, given that the optimal production quality, the viscosity of the components as well as the viscosity of the chemically reacting mixture are temperature dependent. In addition, the viscosity changes in the presence of additives used such as swelling agents fed into the mixing chamber through other nozzles. And, the viscosity of the chemically reacting mixture changes with different mixing ratios of the components.
The average time which elapses for completion of the mixture Preparation also influences the viscosity, and as the elapsed time increases the chemical reactions recurring in the mixture are further advanced as compared with shorter elapsed times.